How this instrument works
A byte is eight bits — the octet width that IBM's System/360 made universal in 1964, eight years after Werner Buchholz coined the name. Attach SI prefixes to that count and you get these two units. Kilo has meant one thousand since the French metric system of 1795; mega has meant one million since the 11th CGPM folded it into the SI in 1960. Neither prefix has ever meant anything else in metrology, so 1 kB is 1000 bytes, 1 MB is 1,000,000 bytes, and one megabyte holds exactly 1000 kilobytes.
Exactly is the operative word. Multiplying by 0.001 discards nothing, because both units are defined by counting bytes rather than by measuring against a physical artefact — a kilobyte is a tally, not a length of platinum. Case carries meaning too: lowercase k is kilo, uppercase K is kelvin, uppercase B is byte and lowercase b is bit. Misread 2500 kb as 2500 kB and your answer is off by a factor of eight.
Both units still earn their keep, at different scales. Kilobytes rule wherever space is rationed: microcontroller flash images, favicons, stylesheets, mail headers, GPS traces. Megabytes are currency for photographs, podcast episodes, firmware bundles and attachment ceilings — Gmail stops you at 25 MB, and a phone camera fills 2 to 5 MB per frame. Anything a build tool reports in kilobytes usually needs comparing against a budget written in megabytes, which is what this sheet does.
- Type your file size into the Kilobytes (kB) field; 2500 is preloaded as a starting point.
- Read the Megabytes (MB) line beneath it — recomputed as you type, with no rounding step in between.
- Working backwards from megabytes? Multiply your MB figure by 1000 and enter that in the Kilobytes (kB) field.
- If your source reads KiB rather than kB, multiply by 1.024 first: those are binary kibibytes, not SI kilobytes.
Worked example — a 2500 kB hero image
Your build report flags one asset: a hero photograph weighing 2500 kB, against a page budget of 2 MB. Enter 2500 in the Kilobytes (kB) field and Megabytes (MB) returns 2.5 — half a megabyte over, and worth another pass through an encoder before it ships.
Check it by hand: 2500 × 0.001 = 2.5, or equivalently 2500 ÷ 1000. Both routes give 2.5 MB, meaning 2,500,000 bytes. Had that report counted in binary kibibytes instead, 2500 KiB would be 2,560,000 bytes — 2.56 MB — and your budget would be blown by rather more than its label suggests.
Questions
Is a megabyte 1000 kilobytes or 1024?
1000, on the decimal SI scale this sheet uses. That 1024 figure belongs to a differently named unit: the mebibyte (MiB), defined by IEC 60027-2 in 1998 as 2^20 bytes, built from 1024 kibibytes (KiB) of 1024 bytes each. One MiB is 1,048,576 bytes against a megabyte's 1,000,000 — roughly 4.9% larger. Storage capacities, network transfers and most file sizes follow decimal counting; memory chip capacities almost always follow binary.
Why does my drive show fewer megabytes than its box promised?
Two rival divisors are at work. A drive labelled 500 GB genuinely holds 500,000,000,000 bytes, counted decimally. Windows then divides by 1024 three times and prints '465 GB' — a binary count wearing a decimal label. Nothing went missing; only arithmetic differs. Apple settled its side of that argument in Mac OS X 10.6 Snow Leopard in 2009 by switching Finder to decimal, which is why one drive can read 500 GB there and 465 GB on Windows.
Is multiplying by 0.001 exact, or is something rounded?
Exact. Both units are defined by counting bytes, and 1000 kilobytes of 1000 bytes each make precisely one million bytes, so this factor carries no uncertainty whatsoever. Any digits that vanish from a displayed result were dropped by formatting, never by arithmetic. Fractional inputs survive intact as well: 2500.4 kB converts to 2.5004 MB.
What is the difference between MB and Mb?
A factor of eight. Uppercase B means byte, lowercase b means bit, so MB is megabytes and Mb is megabits. Internet plans are sold in megabits per second, which is why a 100 Mbps line downloads at roughly 12.5 MB/s rather than 100. That same trap sits one prefix lower: 2500 kb is only 312.5 kB. This instrument works in bytes throughout, so feed it byte figures only.
Should I write kB, KB, or Kb?
kB is what NIST and BIPM prescribe: lowercase k for kilo, uppercase B for byte. Uppercase K denotes kelvin, so KB reads as kelvin-byte, though JEDEC memory standards still spell it that way to mean 1024 bytes and it survives everywhere in practice. Kb is a kilobit, an eighth as large. Where a document mixes all three, check whether it describes storage, memory or a transfer rate before trusting its numbers.
How many kilobytes does a photo or a song take?
A compressed phone photograph runs 2000 to 5000 kB; 2500 kB, as in our worked example above, is a typical 12-megapixel JPEG. A minute of audio at 128 kbit/s costs about 960 kB, so a four-minute track lands near 3.8 MB. Plain HTML pages often weigh 30 to 100 kB, and a favicon a few kB. Those small figures are precisely why kilobytes have not been retired — page-weight budgets are still argued over in hundreds of them.